This invention relates generally to an abnormality detector for nozzle-type dampening systems, and more particularly to an abnormality detector for nozzle-type dampening systems in offset printers for detecting abnormal injection of dampening solution.
In offset printing, a plate having virtually no irregularities and formed in such a manner that its printing image area is lipophilic and its non-image area is hydrophilic is used, both dampening solution chiefly consisting of water and oily ink are fed onto the plate surface, and the ink is allowed to be fed only onto the printing image area by taking advantage of the mutual repellency of water and oil. The dampening system can be roughly divided into two types of mechanism; one is a mechanism where a roller train is provided which extends from the dampening solution reservoir to the plate surface; one rotating roller partly dipped into the solution carries it from the reservoir to the plate surface via the adjacent roller contact surfaces. This allows the dampening solution to be fed in a thin film over the entire axial roller surface.
With this mechanism, however, it is difficult to change the feed of the dampening solution onto different axial areas of the roller. In addition, the ink entering from the plate surface to the dampening solution reservoir via a continuous roller train may contaminate the dampening solution.
Another mechanism is such that the above inconvenience is overcome by dividing the source of the dampening solution into the plate surface and the roller train extending to the plate surface so that the feed of the dampening solution can be changed onto the different areas of the roller by causing the dampening solution to fly toward the plate surface and the roller train extending to the plate surface. Included in this type of mechanism are the nozzle-type dampening arrangements in which the dampening solution is injected through the nozzles, as disclosed in Published Unexamined Japanese Patent Application Nos. Sho-51(1976)-59511, Hei-1(1989)-110146 and Hei-5(1993)330009, for example.
Published Unexamined Japanese Patent Application No. Sho-51(1976)-59511 discloses a dampening arrangement in which the amount of dampening solution regulated by a metering pump is fed to each of a plurality of nozzles; the dampening solution being atomized by a rapid air stream created by the air supplied by a blower. In this dampening arrangement, the metering pump is operated at a desired speed corresponding to the speed of the printer by controlling the operation of its drive motor.
Published Unexamined Japanese Patent Application No. Hei-1(1989)-110146 discloses a dampening arrangement which comprises a pump unit for feeding dampening solution, nozzles for injecting the dampening solution fed from the pump unit, and a controller for controlling the injection of the dampening solution from the nozzles in accordance with the printing speed of the printer. In this dampening arrangement, the injection of the dampening solution is controlled by opening the nozzles for a predetermined time at the timing of injection of the dampening solution calculated from a basic value that is a preset and stored value, an adjustment value that is set in accordance with the printing elements corresponding to each of the injection nozzles, and a corrected value that is a preset and stored value for different printing speed of the printer by relating them to the revolution of a plate cylinder of the printer. That is, control is effected so that a predetermined amount of the dampening solution is injection as the plate cylinder of the printer is rotated by the value calculated above.
Though no specific details have not been disclosed, it is stated in this Patent Application that the duration of injection, that is, the amount of injection of dampening solution may be controlled by changing nozzle opening time or injection pressure, and that the amount of injection of dampening solution may be controlled by changing the opening area of a shutter member provided in front of the nozzles.
Published Unexamined Japanese Patent Application No. Hei-5(1993)-330009 discloses a dampening arrangement comprising a speed detecting means for detecting the printing speed of a printer, a memory for storing the feed of dampening solution to be fed in accordance with printing conditions and the printing speed of the printer, an injecting means having a plurality of nozzles connected by tubing to a source of dampening solution and an air source for continuously injecting onto the plate surface or the surface of the roller coming in contact with the plate surface the dampening solution atomized by a rapid air stream, and a pressure control means provided in the tubing connecting the source of dampening solution and the injecting means for controlling the feeding pressure of dampening solution to the injecting means in accordance with the feed of dampening solution stored in the memory.
In this dampening arrangement, the amount of injection of dampening solution is controlled by setting the feed of dampening solution in accordance with printing speed and printing conditions, such as humidity and temperature, and keeping constant the pressure of dampening solution delivered to the downstream side of the pressure control means by comparing the set feed of dampening solution with the pressure value in the tubing on the downstream side of the pressure control means. In addition, it is also stated in Published Unexamined Japanese Patent Application No. Hei-5(1993)-330009 that a needle valve is provided on the upstream side of each nozzle to fine control the feed of the dampening solution, thereby the feed of the dampening solution can be accurately given.
The prior art pertaining to the checking of the feeding state of dampening solution includes Published Unexamined Japanese Patent Application Nos. Sho-57(1982)-18255, Sho-58(1983)-49252, and Hei-4(1992)-74910.
Published Unexamined Japanese Patent Application Nos. Sho-57(1982)-18255 and Sho-58(1983)-49252 disclose the technology where a roller having a water-containing layer on the surface and electrodes on the inside surface coming in contact with the water-containing layer is provided in a roller train for feeding dampening solution to the plate surface, and the content of dampening solution in the water-containing layer of the roller is checked by obtaining an electric resistance value of the roller water-containing layer or an impedance value across the electrodes.
Published Unexamined Japanese Patent Application No. Hei-4(1992)-74910 discloses the technology where parallel light is shone on the plate surface to which dampening solution is fed, and the reflected light is received and converted into a voltage value based on which the thickness of the dampening solution on the plate surface is calculated.
The prior art, as disclosed in these Japanese Patent Applications, involves the checking of the feeding state of dampening solution on the roller on the mid-stream and downstream of the dampening arrangement, or on the plate surface to which the dampening solution is fed from the dampening arrangement, and has no function for directly checking the feeding state of dampening solution from the nozzles on the upstream side of the dampening arrangement.
The nozzles used in the nozzle-type dampening arrangement are generally of a type where dampening solution is injected in mist form through an extremely narrow opening.
Dampening solution is usually water or water to which a surface tension decreasing agent is added, which tends to include impurities through the generation of salts due to chemical reactions of metallic ions in the solution, the generation of organisms such as bacteria or fine algae, or infiltration of solid particles such as paper powder.
Furthermore, the nozzle-type dampening arrangement, which involves the feeding of the dampening solution slightly pressurized by a pump to the nozzles from the reservoir via tubing in which an appropriate filter is provided, tends to have insoluble matter infiltrated in the feeding path through the filter to the nozzle opening for some reason or other, resulting in the clogging of the nozzle opening preventing the dampening solution from being injected. This leads to poor printing due to the inadequate feeding amount of dampening solution.
It has therefore been desired to create an arrangement for checking the state of dampening solution injected through the nozzles.